146 
species. A similarity in the internal organisation, even if it is very 
close, is not decisive for a variety, for otherwise it would be a simple 
logical necessity to consider for instance the cat as a mere small 
variety of, let us say, the leopard or the rat as a large variety of the 
mouse. Both cat and leopard, and rat and mouse, are internally 
almost absolutely alike and yet they are no varieties but genuine 
species, for cat and mouse preserve their specific peculiarities through 
generations, and never will one of their offspring suddenly be a 
leopard or a rat, nor will the offspring of a leopard or a rat ever 
become cat or mouse, although, hypothetically, a simple enlargement 
(or reduction in size) of their bodies and internal organs combined 
with some changes in colour, &c. would suffice to transform the one 
into the other. With regard to these animals, therefore, (and to 
almost all others as well) the size and some external features, like 
colour, &c. are not considered as determinative of varieties, but of 
species, and it is only the higher classified tory units such as genera, 
sub-families, &c. which become determined by the greater or lesser 
similarity in the internal organisation. Such principles of classification 
are, so far as I can see, universally followed by systematists with 
regard to the larger animals, and I do not see any reason, why they 
should not be applied to parasitic worms also ; so much the more so, 
as careful observation and comparison reveal among them quite a 
similar gradual increase (or decrease) of structural differences as exists 
among the larger animals. I thus hold that it is simply logical and 
consistent to consider the two Opisthorchis sinensis - forms as two 
distinct species rather than as varieties of one species. 
1 , a , Ve * n a ' JOVe explained the reasons for my opinion at some 
ecause the question what in helminthology is to be considered 
as a species still needs being definitely settled. It appears that 
suhif* nU *Mi writeis> zoological and medical, on helminthological 
, • J C , S bl C lng to the old assumption that there exist compara- 
of varviiT ?* eC . ieS P arasdes only, but that these species are capable 
to host 1I J. Slze and organisation to a considerable degree according 
Iwthirt 1 \ U iS my “™ction that th,s view must be 
based f * ^ ° nCC * ^ helminthology is to become an exact science 
fathers. Neither is'V' ° n P rec0nceptions inherited from our fore¬ 
forms us iden justifiable any longer to declare all at once two 
as ,dent,cal simply because they agree in their general structure. 
